1. Field of the Invention
The present invention relates to an electrostatic image developing toner used for electrophotographic image forming.
2. Description of Related Art
It has been required that electrostatic image developing toners used for electrophotographic image forming (hereinafter also simply referred to as “toners”) can be fixed with decreased heat energy in order to increase the printing speed and to reduce the energy consumption of image forming apparatuses. For this reason, further improved low-temperature fixability has been required for toners. For example, in one of such toners known in the art, a crystalline polyester resin having a sharp-melting property is introduced to the toner as a binding resin so as to decrease the glass transition point or the melt viscosity of the binder resin.
Specifically, for example, it is known to use a binder resin that is a mixture of an amorphous resin and a crystalline polyester resin having high miscibility with the amorphous resin. Such combined use of a crystalline polyester resin can improve the low-temperature fixability since the crystalline polyester resin serves as a plasticizer in heat fixing (e.g. see JP 4729950B).
However, a problem with such toners is poor heat-resistant storage stability, which is due to plasticization of toner particles caused by mutual dissolution between the amorphous resin and the crystalline polyester resin.
To cope with the poor heat-resistant storage stability, JP 4729950B discloses a method for reducing the decrease in heat-resistant storage stability by adjusting the glass transition point of the toner.
Another method for improving the heat-resistant storage stability is to form a shell layer composed of a low-miscible amorphous resin having low miscibility with a crystalline polyester resin on a core particle composed of a high-miscible amorphous resin and the crystalline polyester resin (e.g. see JP 4742936B).
However, such toners may lack in image fixing strength since the binder resin is less fusible in heat fixing due to the high glass transition point of its amorphous resin, and the shell layer, which is formed on the surface of each toner particle, is not plasticized but remains stiff in heat fixing. Therefore, a fixed image may be lack in fixing strength. As a result, these toners cannot be a perfect solution for poor heat-resistant storage stability.